Circuits for the generation of a voltage reference, also simply indicated as voltage references, are widely used in the integrated circuits for the most varied needs.
These circuits supply, in particular, at least one electric quantity having a high accuracy and stability that can be used in general as reference in several circuit blocks, such as for example, analogue/digital converters, voltage regulators, detection and/or measurement circuits, etc.
A voltage reference should thus be strong for the applications it is intended for and in particular be characterized by a good thermal stability and by a good noise rejection, so as to supply a constant output voltage value independent from the variations of the supply voltage and of the working temperature of the integrated circuit comprising it.
To this purpose, circuits are commonly used for generating a voltage reference of the band-gap type, or more simply band-gap generators, wherein, the potential jump of the silicon prohibited band (about 1.1 eV) is exploited for generating an accurate voltage reference independent from the working temperature.
In particular, such a band-gap generator arises from the realization that a voltage VBG almost independent from the working temperature can be obtained in a simple way by means of a bipolar transistor by implementing the following equation:VBG=VBE+nVT   (1)VBG being the voltage reference independent from the temperature, or of band-gap, VBE being the voltage between the base and emitter terminals of the bipolar transistor used, VT being the thermal voltage (equal to kT/q, k being the Bolzmann constant, T being the absolute temperature and q being the electron charge) and n being a multiplicative parameter calculated to obtain the desired compensation of the variations in temperature of the voltage VBE.
The voltage VBE between base and emitter of a bipolar transistor decreases when the temperature increases (˜−2.2 mV/° C.@T=300° K), while the thermal voltage VT is proportional to the temperature itself. In other words, a voltage (VBE) is to be compensated which decreases with the absolute temperature, i.e., it is CTAT (Complementary To Absolute Temperature) with a corrective coefficient (nVT) which is proportional to the absolute temperature or PTAT (Proportional To Absolute Temperature).
To obtain a voltage reference independent from the temperature one determines the value of the parameter n for which the derivative of the band-gap voltage VBG, with respect to the temperature, is equal to zero considering a temperature T=T* equal to a desired working temperature. For example if a null variation of the band-gap voltage reference VBG is to be obtained at the temperature of 27° C., a value of about 1.26V for VBG is found, the voltage VBE being at environment temperature equal to about 0.6V and the parameter n equal to about 26.
A band-gap generator may be realized in full CMOS technology realising the bipolar transistors by means of parasitic diodes. A possible implementation using an operational amplifier is shown in FIG. 1.
In particular, FIG. 1 shows a generator 1 of a band-gap voltage reference VBG. This generator 1 comprises an operational amplifier 2 inserted between a first and a second voltage reference, in particular a supply voltage reference VDD and a ground GND.
The operational amplifier 2 has a first input terminal T1, in particular an inverting one (−), and a second input terminal T2, in particular a non inverting one (+), as well as an output terminal, corresponding to the output terminal OUT of the generator 1, where the band-gap voltage reference VBG is supplied.
The generator 1 also comprises a bipolar stage 3 inserted between the output terminal OUT of the operational amplifier 2 and the ground GND and comprising a first Q1 and a second bipolar transistor Q2, as well as a first R1, a second R2, and a third resistive element R3.
More in particular, the first bipolar transistor Q1 is inserted between the second input terminal T2 of the operational amplifier 2 and the ground GND and has a control or base terminal coupled to the base terminal of the second bipolar transistor Q2 and both coupled to ground (both the bipolar transistors are diode-connected). The bipolar transistor Q2 is also coupled, through the first resistive element R1, to the first input terminal T1 of the operational amplifier 2 as well as to the ground GND.
The second input terminal T2 of the operational amplifier 2 is also feedback connected to its output terminal OUT, by means of the second resistive element R2 and the first input terminal T1 of the operational amplifier 2 is similarly feedback connected to its output terminal OUT, by means of the third resistive element R3.
It is to be noted that the operational amplifier 2 performs the double function of realizing a current proportional to the thermal voltage VT and of ensuring the output supply of a band-gap voltage reference VBG with low impedance, which is desirable, when the generator 1 should supply current.
Thanks to the presence of the operational amplifier 2 it is possible to assume that the voltage values on its input terminals T1 and T2 are identical (V+=V−), by putting AE2=kAE1, AE2, AE1 being the areas of the emitter terminals of the first and second bipolar transistors Q1 and Q2, respectively and k being a suitable project parameter calculated to obtain the desired temperature compensation.
Observing moreover that R2*IC1=R3*IC2, R2 and R3 being the resistive values of the second and third resistive elements R2 and R3, respectively, and IC1, IC2 the collector currents of the first and second bipolar transistors Q1 and Q2, respectively, the following is obtained:
                              I                      C            ⁢                                                  ⁢            2                          =                                            V              T                                      R              2                                ⁢                      ln            ⁡                          (                                                                    R                    3                                                        R                    2                                                  ⁢                k                            )                                                          (        2        )            
Wherefrom the expression of the band-gap voltage reference VBG is easily derived:
                              V          BG                =                              V                          EB              ⁢                                                          ⁢              1                                +                                                    R                3                                            R                1                                      ⁢                          V              T                        ⁢                          ln              ⁡                              (                                                                            R                      3                                                              R                      2                                                        ⁢                  k                                )                                                                        (        3        )            VEB1 being the voltage between the base and emitter terminals of the first bipolar transistor Q1 and R1, R2, R3 the resistive values of the first, second and third resistive elements.
It is to be noted that the minimum value of the supply voltage reference VDD of the generator 1 under examination depends on the effective physical realization of the operational amplifier 2, but it results in any case limited below by the reference voltage value calculated for having a null variation at the environment temperature, equal to about 1.26V, as above indicated.
The generator 1 realized by means of the operational amplifier 2 and shown in FIG. 1 cannot thus be used in applications having supply voltages lower than about 1.3V.
It is also possible to modify the generator 1 to adapt it to applications with supply voltage lower than 1.3V and to obtain the generator 5 shown in FIG. 2, also inserted between a first and a second voltage reference, in particular a supply voltage reference VDD and a ground GND and having an output terminal OUT where the band-gap voltage reference VBG is supplied.
The generator 5 also comprises an operational amplifier 2 having a first input terminal T1, in particular an inverting one (−), and a second input terminal T2, in particular a non inverting one (+), as well as an output terminal OUT.
The generator 5 further comprises an input stage 6 inserted between the input terminals, T1 and T2, of the operational amplifier 2 and the ground GND, in turn including a first Q1 and a second bipolar transistor Q2, as well as a first R1, a second R2, and a third resistive element R3.
More in particular, the first bipolar transistor Q1 is inserted, in series with the first resistive element R1, between the first input terminal T1 of the operational amplifier 2 and the ground GND and has a control or base terminal coupled to the ground GND.
Similarly, the second bipolar transistor Q2 is in turn inserted, in series with the second and the third resistive element R2, R3, between the second input terminal T2 of the operational amplifier 2 and the ground GND and has a control or base terminal coupled to the ground GND.
The generator 5 also comprises a current mirror 7, inserted between the supply voltage reference VDD and an inner circuit node X′ and coupled to the input terminals T1, T2 of the operational amplifier 2, as well as with its output terminal OUT and including a first, a second and a third MOS transistor, M1, M2 and M3 as well as a first capacitor C1.
More in particular, the first MOS transistor M1 is inserted between the supply voltage reference VDD and the first input terminal T1 of the operational amplifier 2 and has a control or gate terminal coupled to the control or gate terminal of the second MOS transistor M2, and both coupled to the output terminal OUT of the operational amplifier, the second MOS transistor M2 being in turn inserted between the supply voltage reference VDD and the second input terminal T2 of the operational amplifier 2. Similarly the third MOS transistor M3 is inserted between the supply voltage reference VDD and the inner circuit node X′ and has the control or gate terminal coupled to the output terminal OUT of the operational amplifier 2 as well as with the bulk terminal of the second MOS transistor M2.
Finally, the first capacitor C1 of the current mirror 7 is inserted between the supply voltage reference VDD and the output terminal OUT of the operational amplifier 2.
In this way, the current mirror 7 is able to supply the inner circuit node X′ with a value of current IP1 proportional to the current flowing in the first bipolar transistor Q1 of the input stage 6.
The generator 5 also comprises an output stage 8 inserted between the inner circuit node X′ and the ground GND and coupled to the output terminal OUT′ of the generator 5 and comprising a third bipolar transistor Q3, a fourth and a fifth resistive element R4 and R5 and a second capacitor C2.
More in particular, the fourth resistive element R4 and the third bipolar transistor Q3 are inserted, in series with each other, between the inner circuit node X′ and the ground GND, the third bipolar transistor Q3 also having a control or base terminal in turn coupled to the ground GND. Similarly, the fifth resistive element R5 and the second capacitor C2 are inserted, in parallel to each other, between the inner circuit node X′ and the ground GND.
It is to be noted that the voltage values on the input terminals T1 and T2 of the operational amplifier 2 being equal (V+=V−) and having:                AE2=nAE1, R1=R3, IP1=k1IP being:            AE2, AE1 the areas of the emitter terminals of the first and second bipolar transistors Q1 and Q2, respectively, of the input stage 6 and n a suitable multiplicative coefficient calculated to obtain the desired compensation in temperature,    R1, R3 the resistance values of the first and of the second resistive element of the input stage 6, and    IP, IP1 the current values flowing in the first bipolar transistor Q1 of the input stage 6 and in correspondence with the inner circuit node X′ at the output of the current mirror 7, respectively, and k1 a suitable multiplicative coefficient introduced by the dimensional ratio of the transistors M1 and M3 of this current mirror 7 with simple mathematical expressions, it is possible to obtain the following expression of the band-gap voltage reference VBG:
                              V          BG                =                                            R              5                                                      R                5                            +                              R                4                                              ⁢                      (                                          V                                  EB                  ⁢                                                                          ⁢                  3                                            +                                                                    R                    4                                                        R                    2                                                  ⁢                                  V                  T                                ⁢                                  K                  1                                ⁢                                  ln                  ⁡                                      (                                                                  I                                                  S                          ⁢                                                                                                          ⁢                          2                                                                                            I                                                  S                          ⁢                                                                                                          ⁢                          1                                                                                      )                                                                        )                                              (        4        )            being:    R2 the resistance value of the second resistive element of the input stage 6, R4, R5 the resistance values of the fourth and fifth resistive elements of the output stage 8,    VEB3 the voltage value between the base and emitter terminals of the third bipolar transistor Q3 of the output stage 8; and    IS1, IS2 the inverse saturation current values of the first and second bipolar transistors Q1 and Q2, respectively.
It thus occurs that the resistive elements R1 and R3 are suitable for ensuring that signals at the input of the operational amplifiers 2 are adequate also at high temperatures, when the voltage value between the base and emitter terminals VEB of the bipolar transistors is low.
In fact, it is to be noted that the differential pair with which the operational amplifier is realized (not shown in the figure), for applications with low supply voltage values, should be of the n-channel type since a pair of p-channel transistors would be off for values of the supply voltage below about 1.4V. The resistive elements R1 and R3 put in series with the bipolar transistors Q1 and Q2 have the function of allowing a correct operation range at the input terminals T1 and T2 of the operational amplifier 2, substantially increasing by a certain amount the voltage value at the input terminals T1 and T2 of the operational amplifier 2, since the voltage VBE of these bipolar transistors Q1 and Q2 at high temperatures decreases too much for ensuring the turn-on of the n-channel transistors.
In this way, the generator 5 is able to offer good performances down to values of the supply voltage equal to about 1.1V.
However, for lower supply voltage values, and especially at low temperatures, when the voltage value between the base and emitter terminals VEB of the bipolar transistors is high, it may occur that the first and the second MOS transistors M1 and M2 of the current mirror 7 operate with a very low voltage value between the source and drain terminals Vds, and in particular quite different from the voltage value between the source and drain terminals Vds of the third MOS transistor M3, this latter voltage being considered constant for the whole temperature range.
These different operative conditions may cause mirroring errors of the currents, which may result in a poor behavior of the generator 5 when the temperature varies.